A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
Commonly, and especially in scanners, the substrate and the patterning device are supported by a corresponding support structure, wherein the support structure is movable with respect to a frame. The frame may be a base frame or a metrology frame. The support structure is positioned with respect to the frame by a positioning system including a motor and a control system to drive the motor. Motors in general can be characterized by a parameter known as “motor constant”. The motor constant defines a relation between motor input and motor output. Examples of a motor input are current, voltage, frequency, etc. Examples of a motor output are velocity, acceleration, force, torque, etc. Most of the time, the motor constant is assumed to be a constant parameter, explaining the name motor constant. However, in practice, motors exhibit a motor position dependent motor constant which can be divided into a constant average portion and a motor position dependent portion. Thus, when a person skilled in the art assumes a constant motor constant, he/she actually assumes the motor constant to be the constant average portion. The motor constant is motor position dependent due to for instance structural inaccuracies, tolerances, and/or deficiencies or inhomogeneous properties, such as material properties. Possible causes for the motor position dependency of the motor constant of an electromagnetic motor are:                1. variations in gap between magnets;        2. variations in size of the magnets;        3. variations in magnetic field strength and magnetization direction/orientation of the magnets;        4. non-ideal winding of coils;        5. different winding of different coils;        6. non-ideal commutation; and        7. local temperature effects.        
As used hereinafter, the motor position is a relative position of different motor parts with respect to each other, e.g. a rotor with respect to a stator. If the motor is a rotational motor, the motor position after a 360° rotation is the same. It is possible that there is a one on one relation with a position of an object positioned by the motor. But especially after a rotation of 360° of a rotational motor, the object may be in a different position, while the motor position is the same. In an electromagnetic planar motor, the motor position is a position of one or more coils (also called actuator) with respect to an array of permanent magnets or the position of (an array of) magnets with respect to the coil(s). Note that the motor position is not limited to a one degree of freedom case, but can also be a multi degree of freedom position. With a degree of freedom is meant a translation (X, Y, or Z) or a rotation (Rx, Ry, or Rz)
The motor position dependent portion of the motor constant introduces disturbances in the positioning system which negatively influence the position accuracy of the positioning system. In many applications, the assumption of the motor constant being a constant parameter suffices, since the disturbances introduced, or the effect of the introduced disturbances, by the motor position dependent portion of the motor constant are most of the time small compared to a desired position accuracy of the positioning system. The motor position dependent portion can therefore be neglected. Also, the disturbances are only present while moving the motor from one motor position to another motor position and do not affect the position accuracy of the positioning system when the motor is stationary, i.e. the different motor parts do not move with respect to each other.
In a lithographic apparatus, and especially in so-called scanners, the position accuracy during movement of the motor is becoming more and more important. In a scanner, each target portion is irradiated by scanning the pattern through the radiation beam in a given direction while synchronously scanning the substrate parallel or anti-parallel to this direction. Scanning of the substrate and/or pattern is done by moving the corresponding support structure with respect to the frame using a positioning system. With an increasing required positioning accuracy of the positioning system during scanning, the motor position dependent portion of the motor constant becomes a non-negligible disturbance factor. The motor position dependent portion of the motor constant not only affects the position accuracy of each positioning system, but also affects the mutual position accuracy between the substrate and pattern while scanning. A disturbance in the mutual position may result in overlay errors and/or imaging problems of the lithographic apparatus.